It is desirable that the beam pointing direction of large steerable microwave antennas be known accurately. A measure of this accuracy is known in the art as "pointing error," and is defined as the angular difference between the total angle pointing direction indicated by antenna instrumentation and the direction toward the actual maxima of the radio source being tracked.
As a practical matter, the position indicator, which generally takes the form of an angle transducer, which senses angular pointing of the antenna must be connected physically to the structure of the antenna through mechanisms which are not exactly indicative of the true pointing vector due to distortions of the entire structure. Some error causing distortions such as those caused by wind loads and thermal gradients are classed as "random;" others are "systematic" because they can be predicted as a function of known operating parameters.
For all antenna axis which rotate about a nonvertical axis (e.g., elevation axis of common Az/El systems) gravity is in a changing relation to the structure as axis rotation takes place. Each structural element distorts with respect to other elements according to mass and stiffness characteristics. The antenna microwave optics distort causing the vector representing the maximum signal received to distort with respct to all structural elements and in particular to the drive shaft of the position indicator or transducer used to indicate antenna pointing.
The classical solution to this problem has been to make the structure stiff enough to reduce the overall deflection to a magnitude small with respect to the allowable pointing error. The costs associated with this approach are high since the analysis to determine the deflection characteristics of candidate antenna structures is tedious and since the required stiff structure is very heavy, thus expensive.
Another method has been explored to reduce pointing error when the structural design was not governed by gravity deflection. The system was calibrated to establish the relation between pointing error and indicated elevation pointing angle so that a correction to the position signal generated by the transducer could be made to its output according to the uncorrected output. The computer equipment to accomplish this function was costly and contributed to the unreliability and complexity of the system.
Prior art patents representative of methods of preventing deflections in the antenna structure to reduce pointing error include:
U.S. Pat. No. 3,239,839 issued to J. Banche et al on Mar. 8, 1966, for "Antenna Reflector Surface Contour Control." Banche et al shows a method of compensation with levers whose moment arms change with the orientation of the antenna and which apply forces to the dish so as to compensate for the distortion of the dish due to antenna orientation.
U.S. Pat. No. 3,153,789 issued to E. L. Ashton on Oct. 20, 1964, for "Large Aperture Steerable Trunnion - Mounted Paraboloidal Antenna." Ashton shows a large antenna where distortions due to gravitational and/or wind stresses are compensated by pre-distorting certain struts so that their lengths either do not change or the lengths compensate for the defocusing of the antenna due to stresses. A position indicator is shown in FIGS. 36 through 40 and described in column 13 and 14.
U.S. Pat. No. 2,408,825 issued Oct. 8, 1946, to R. H. Varian et al for "Object Detecting and Locating Device." Varian et al shows the use of torque arms to reduce vibration in scanning dish antennas.